Fix AttributeError in OAuth version detection.
Fix AttributeError in OAuth version detection.

Old versions of OAuth didn't have _version attribute, so Twython crashes
on them with AttributeError.

This version first checks if there is _version attribute. If no,
obviously it's an old version.

ryanmcgrath/twython,Oire/twython,akarambir/twython,kracekumar/twython,Fueled/twython,Devyani-Divs/twython,fibears/twython,mgrouchy/twython,vivek8943/twython,joebos/twython,Hasimir/twython,ping/twython